This invention generally relates to processes for treating high cycle rate items to improve durability, and more particularly to cryogenic treatment of clutch disc springs processed cryogenically to improve durability and life.
In a land vehicle, such as an over-the-road truck or an automobile, a clutch engages and disengages the transmission with and from the engine. Engaging the clutch transfers power from the engine to the transmission and wheels, while disengaging the clutch allows the engine to rotate independently, without driving rotation of the transmission. As is known, clutches are also employed with engines not used to power land vehicles. In a typical design, a flywheel mounts on the engine and rotates conjointly with the engine""s crankshaft while a clutch disc of the clutch rotates with the transmission. A clutch disc is arranged for flatwise engagement with the flywheel when a pressure plate urges the clutch disc against the flywheel. When the clutch disc engages the flywheel and pressure plate, the friction between the clutch disc, flywheel and pressure plate eventually causes the clutch disc to rotate conjointly with the flywheel. Because the clutch disc and flywheel can rotate at significantly different rates when disengaged, there is a significant initial torque when the two are re-engaged. It is necessary to protect the engine and transmission from being damaged by this torque.
Clutch discs may be either rigid or flexible. Flexible clutch discs, the focus of the present invention, typically comprise two parts, an inner hub and an outer friction disc. A plurality of clutch disc springs typically engage the inner hub and the outer friction disc. These springs dampen the impact of engaging the clutch disc with the flywheel and pressure plate. After repeated use, these dampening springs can become weak from fatigue and fail. Moreover, abuse of such springs, by engaging the transmission in an inappropriate-gear (where the flywheel speed greatly differs from transmission speed) or by partially engaging the clutch linkage for extended periods (i.e., riding the clutch), can lead to premature stress and fracture. When a clutch disc spring fails, the functioning of the clutch greatly deteriorates because the spring is unable to absorb the shock of the clutch disc engaging the flywheel and pressure plate. Moreover, the time and expense required to replace such a clutch disc spring or entire clutch disc is undesirable. Therefore, it is the aim of the present invention to provide a clutch disc spring that exhibits improved life and durability to avoid the expense and time associated with failed springs.
Among the several objects and features of the present invention may be noted the provision of a clutch disc spring and clutch incorporating the clutch spring that exhibits improved durability and life; the provision of a method of treating clutch disc spring that significantly improves the durability and life of such a spring; and the provision of such a method that treats other high cycle ferrous items for improved mechanical properties.
Generally, a method of treating clutch disc springs to increase the average number of cycles to failure is disclosed. The method comprises the step of providing at least one clutch disc spring, wherein the spring is formed from steel. The spring is then heated to a temperature in a range from about 1400 degrees Fahrenheit to about 1700 degrees Fahrenheit. The spring is then rapidly quenched to a temperature in a range from about 70 degrees Fahrenheit to about 550 degrees Fahrenheit. The method then tempers the spring to a temperature in a range from about 300 degrees Fahrenheit to about 800 degrees Fahrenheit. The spring is then cooled slowly to a cryogenic treatment temperature of about xe2x88x92300 degrees Fahrenheit over a period of at least about 7 hours. The spring is further maintained at said cryogenic treatment temperature over a period of at least about 15 hours for converting any trace amounts of austenite within the spring into martensite so that the spring is substantially free from austenite. The method then warms the spring slowly from the temperature range over a period of at least about 7 hours. The spring is then tempered at a temperature of about 300 degrees Fahrenheit over a period of at least about one hour.
In another embodiment, a method of treating clutch disc springs is disclosed wherein a spring is provided having been formed from steel partially comprised of trace amounts of austenite. The spring is then cooled slowly from an ambient temperature range to a cryogenic treatment temperature in a range from about 250 degrees Fahrenheit to about xe2x88x92350 degrees Fahrenheit over a period of at least about 7 hours. The spring is then maintained within the cryogenic treatment temperature range over a period of at least about 15 hours. The spring is then warmed from the cryogenic treatment temperature to the ambient temperature range over a period of at least about 7 hours. The spring is then tempered at a temperature in a range from about 280 degrees Fahrenheit to about 400 degrees Fahrenheit over a period of at least about one hour.
In yet another embodiment, a method of treating ferrous items subjected to high cycle rates is disclosed. The method comprises steps similar to those set forth above in the previous embodiment.
Other objects and features of the present invention will be in part apparent and in part pointed out hereinafter.